Linker Error when building for QNX

[Contilehpat]

Describe the bug
When building for QNX, a linker error appears, which states that libpthread was not found. In QNX, there is no explicit pthread library. However, pthreads are supported.

To Reproduce
Compile the package with the following cmake calls:
cmake -DPAHO_BUILD_SAMPLES=OFF -DPAHO_BUILD_EXAMPLES=OFF -DPAHO_BUILD_SHARED=ON -DPAHO_BUILD_STATIC=OFF -DPAHO_BUILD_TESTS=OFF

Expected behavior
Compiles as inteded

Screenshots

Log files

** Environment (please complete the following information):**

• OS: Ubuntu Linux 20.04
• Version [e.g. 22]
• QNX 7.0 environment

Additional context
Compilation works with these modifications in src\CMakeLists.txt:
ELSEIF (CMAKE_SYSTEM_NAME MATCHES "QNX") SET(LIBS_SYSTEM c )